1. Field of the Invention
The invention relates to an optical device, in particular a length- or an angle-measuring device, which detects changes in an interference pattern of at least two light beams which are capable of interference and which are brought to interference by a diffraction grid.
2. Description of Related Art
Length and angle measuring devices (also referred to herein as "arrangements") of this type, are known from many references, for example, from Spies, U.S. Pat. No. 4,792,678 and Pettigrew, U.S. Pat. No. 4,776,701, the entire contents of both of which are incorporated herein by reference.
Such devices operate, generally, by measuring changes in interference pattern of partial beams. In a position measuring arrangement disclosed by Pettigrew, an index grid is a phase grid wherein the phase relation between zero order and first order can be varied by varying the proportion of ridge to groove and the phase depth of castellations of the phase grid.
According to Lee, High Efficiency Multiple Beam Gratings, APPLIED OPTICS, Vol. 18, No. 13 (Jul. 1, 1979), pp. 2152-2158, Damman et al. demonstrated that intensities of diffracted orders could be made uniform by controlling the grating profile (Damman et al., OPT. COMMUN. 3,312 (1971). The entire contents of Lee and Damman et al. are incorporated herein by reference.
However, previously known measuring systems based on the dual beam interference of partial beams of varied diffraction order have some disadvantages. For example, such systems use interfering partial light beams which have different degrees of diffraction effect. This leads to an unsatisfactory degree of modulation of the electrical signals generated by the detectors detecting the changes in the interference pattern.